<html>  
    <head> 
    	<meta charset="utf-8"> 
        <title>验证码</title>  
        <style type="text/css">  
            #code{  
                font-family:Arial;  
                font-style:italic;  
                font-weight:bold;  
                border:0;  
                letter-spacing:2px;  
                color:blue;  
            }
        </style>  
    </head>  
    <body>  
        <div>  
	        <input type = "text" id = "input" value="" />  
	        <input type = "button" id="code" onclick="createCode()"/>  
	        <input type = "button" value = "验证" onclick = "validate()"/>  
    	</div> 
    </body>  
    <script type="text/javascript">
	//设置一个全局的变量，便于保存验证码
    var code;
    function createCode(){
        //首先默认code为空字符串
        code = '';
        //设置长度，这里看需求，我这里设置了4
        var codeLength = 4;
        var codeV = document.getElementById('code');
        //设置随机字符
        var random = new Array(0,1,2,3,4,5,6,7,8,9,'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R', 'S','T','U','V','W','X','Y','Z');
        //循环codeLength 我设置的4就是循环4次
        for(var i = 0; i < codeLength; i++){
            //设置随机数范围,这设置为0 ~ 36
             var index = Math.floor(Math.random()*36);
             //字符串拼接 将每次随机的字符 进行拼接
             code += random[index]; 
        }
        //将拼接好的字符串赋值给展示的Value
        codeV.value = code;
    }

    //下面就是判断是否== 的代码，无需解释
    function validate(){
        var oValue = document.getElementById('input').value.toUpperCase();
        if(oValue ==0){
            alert('请输入验证码');
        }else if(oValue != code){
            alert('验证码不正确，请重新输入');
            oValue = ' ';
            createCode();
        }else{
            window.open('http://www.baidu.com','_self');
        }
    }
    //设置此处的原因是每次进入界面展示一个随机的验证码，不设置则为空
    window.onload = function (){
        createCode();
    }
    </script>
</html> 